1. Field of the Invention
Embodiments of the present invention relate to loudspeaker protection circuitry. More particularly, embodiments of the present invention relate to a low-cost, sonically transparent, multi-stage loudspeaker protection circuit that protects a loudspeaker device from RMS and short-duration transient over-voltage conditions while accommodating adjustable threshold and dynamic attack timing.
2. Description of the Related Art
It is often desirable to protect a loudspeaker from excessive voltage and current conditions which may lead to permanent damage to the loudspeaker. It is also desirable to allow permissible voltages and currents to pass to the loudspeaker without significant attenuation, distortion, or filtration. Due to the variety of loudspeaker transducer designs, voltage and current limits vary significantly; however, typical transducers can handle large power levels for a short duration and reduced power levels for longer durations, as will be discussed later. Many attempts have been made to protect loudspeaker transducers from over-voltage and/or over-current conditions through protective circuitry. Unfortunately, these attempts have failed to adequately protect the transducer while allowing all permissible voltages and currents to pass unaltered.
Existing protection circuits have generally used a combination of actuating devices and attenuating devices. Actuating devices have been selected to actuate during over-voltage or over-current conditions and include such devices as thermistors, lamps, relays, fuses, diodes, etc. Some actuating devices, such as lamps, thermistors, diodes, etc., are considered self-actuating in that they do not require detection and/or triggering circuitry. Unfortunately, self-actuating devices are not adjustable and actuation threshold can vary significantly depending on ambient temperature and/or production tolerances. Other actuating devices, such as relays, are considered controlled actuating devices because they require detection and triggering circuitry to control the actuation. Attenuating devices have been used to attenuate, or reduce the unwanted voltages or currents and include such devices as resistors, lamps, diodes, thyristors, etc. Some devices, such as lamps, are both actuating and attenuating devices. In other words, such a device will actuate at certain voltages and currents by becoming a resistive attenuating element. Attenuating devices can further be grouped as variable attenuators or fixed attenuators. A resistor with constant impedance would be considered a fixed attenuator, while a lamp with current & heat dependant impedance would be considered a variable attenuator.
Referring to FIG. 1A, two aspects of typical loudspeaker power handling performance are presented; required attenuation and time duration. Typical loudspeakers have a rated power handling specification, below which the transducer will operate without damage illustrated by the dotted line, 44. The left axis, 40, corresponds to an attenuation value in decibels (dB). The solid line, 42, represents the loudspeakers required attenuation to sustain proper operation without damage to the transducer. The right vertical axis, 46, corresponds to time in seconds (sec). The dashed line, 48, represents the loudspeakers power handling as a function of time duration in seconds. The common horizontal axis, 50, represents increasing power. As evident in FIG. 1A, the loudspeaker requires increasing attenuation as the input power level exceeds the rated power of the loudspeaker. Also, as the input power increases, the duration of time within which the loudspeaker will operate without damage steadily decreases. Effective loudspeaker protection should seek to provide adequate attenuation above the rated power handling of the transducer, and should control the duration of power levels in excess of the rating. Additionally, effective loudspeaker protection should seek to allow all power levels below the transducer rating to pass unaltered, i.e. minimal attenuation, filtration, and distortion.
Referring to FIG. 1B, required attenuation and time duration plots of a typical loudspeaker are overlaid with a typical self-actuating, self-attenuating lamp. The lamp's attenuation is represented by the solid line, 52, and the time response to reach the nominal attenuation is represented by the dashed line 54. As evident in FIG. 1B, the lamp self-actuates and attenuates before the loudspeaker's power handling rating, 44, and begins a linear increase in attenuation. Unfortunately, the lamps attenuation plateaus and is significantly less than what the loudspeaker requires to maintain damage-free operation. Shaded region 56 illustrates the damage region in which the loudspeaker would receive more power than the specified rating. The lamps time response is very fast, as evident by dotted line 54, clearly faster than the required response time of the loudspeaker, 48. Unfortunately, the lamp's excessive speed will clamp transient power levels quicker than required resulting in a less-musical solution. Lamps also have a nominal impedance even when they are not actuated or lighting, which results in a measurable insertion loss. Additionally, lamps have a maximum power rating and the filament can be damaged upon over-powering the device, which greatly limits the operational power range of circuits that incorporate lamps without subsequent filament protection.
Referring to FIG. 1C, required attenuation and time duration plots of a typical loudspeaker are overlaid with a typical self-actuating thermistor (usually a Positive Temperature Coefficient device, PTC). The PTC attenuation is represented by the solid line, 52, and the time response to reach the nominal attenuation is represented by the dashed line 54. As evident in FIG. 1B, the PTC actuates slightly before the loudspeaker's power handling rating, 44, and steps quickly in attenuation. While the PTC does offer adequate attenuation, the fast-acting step attenuation response is not musical and easily detected by the human ear. The PTC time response is very slow, as evident by dotted line 54, and is clearly slower than the required response time of the loudspeaker, 48. Shaded region 56 illustrates the damage region in which the loudspeaker would receive longer power durations than the specified rating. Unfortunately, while selecting smaller PTC devices will speed the time response, the actuation threshold is typically much less than the desired power rating of the loudspeaker. Additionally, PTC devices will remain actuated with a small amount of trickle current, leading to poor release and recovery performance. PTC actuation thresholds will also vary greatly depending upon the ambient temperature, greatly limiting the effective operational temperature range of circuits incorporating such devices. Because of these problems, designers have great difficulty finding a single PTC device that meets all of the desired requirements with respect to time, attenuation, actuation thresholds, and release performance.
Referring to FIG. 1D, required attenuation and time duration plots of a typical loudspeaker are overlaid with a typical relay and a single lamp attenuator. The attenuation characteristic, solid line 52, is the same as a single lamp; however, the lamp is not allowed to actuate below the power rating of loudspeaker. Unfortunately, the inadequate attenuation of the lamp at higher power levels remains a problem and allows operation in the damage region, 56. Replacing the lamp attenuator with a constant impedance attenuator results is a large stepped attenuation, which is not musical and easily detected by the human ear. The relay time response is very fast, as evident by dotted line 54, clearly faster than the required response time of the loudspeaker, 48. Unfortunately, the excessive speed will clamp transient power levels quicker than required, again making the protection topology less musical. Additionally, typical relay designs have suffered from actuation chatter wherein the relay actuates and releases rapidly when the input signal is crossing the relay coil threshold. Such chatter degrades the life of the relay contacts significantly.
Existing designs that incorporate thyristors or clamping diodes are effective in limiting peak voltages; however, excessive currents exist when clamping and can result in damage to the clamping device, the driving amplifier, or the passive crossover connected thereto. Such crow-bar, clamping techniques result in non-linear loading on the driving device and are not acceptable for protection circuits that are required to connect to a variety of different amplifiers and/or required to connect to the output of a passive crossover filter requiring proper termination.
In summary, existing protection circuits have suffered from the following problems: uncontrolled response time (excessively fast or slow), high insertion loss, frequency selectivity, abrupt stepped actuation, non-linear loading, inadequate peak voltage and current protection, limited operational power range, and actuation chatter.